Optical device and method of in situ treating an euv optical component to enhance a reduced reflectivity

ABSTRACT

The present invention relates to an optical device and a method of in situ treating an optical component ( 2, 6, 13 ) reflecting EUV and/or soft X-ray radiation in said optical device, said optical component ( 2, 6, 13 ) being arranged in a vacuum chamber ( 14 ) of said optical device and comprising one or several reflecting surfaces ( 3 ) having a top layer of one or several surface materials. In the method, a source ( 1, 5 ) of said one or several surface materials is provided in said chamber ( 14 ) of said optical device and surface material from said source ( 1, 5 ) is deposited on said one or several reflecting surfaces ( 3 ) during operation and/or during operation-pauses of said optical device in order to cover or substitute deposited contaminant material and/or to compensate for ablated surface material.

CROSS REFERENCE TO PRIOR APPLICATIONS

This application is a divisional application of U.S. Ser. No.12/602,798, filed on May 6, 2010 which is a U.S. National Phaseapplication under 35 U.S.C. §371 of International Application No.PCT/IB2007/052224, filed on Jun. 12, 2007. These applications are herebyincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to an optical device for EUV and/or softX-ray radiation comprising at least one optical component in a vacuumchamber, which optical component has one or several reflecting surfaceswith a top layer of one or several surface materials. The invention alsorelates to a method of in situ treating such an optical componentreflecting EUV and/or soft X-ray radiation in an optical device in orderto enhance a reduced reflectivity of the optical component.

BACKGROUND OF THE INVENTION

The present invention refers to the field of optical devices for thespectral range of extreme ultraviolet (EUV) and/or soft X-ray radiationcomprising optical components with reflective surfaces for reflectingthe EUV and/or soft X-ray radiation. Such optical devices are required,for example, for EUV lithography, in which grazing incidence mirrorsand/or multilayer mirrors are arranged in a vacuum chamber between theradiation source and the wafer substrate to be irradiated. Typicalmaterials used for the reflecting surfaces of the grazing incidencemirrors are for example ruthenium (Ru), palladium (Pd) or molybdenum(Mo). Multilayer mirrors for the above spectral range, which are suitedfor vertical or near vertical incidence, typically comprise acombination of layers of molybdenum and silicon (Si). Often also a toplayer of ruthenium is applied for protecting the underlying layers.

A problem mainly arising during operation of optical devices with suchreflecting optical components is the decrease of reflectivity over time.This reduction in reflectivity can be caused by contaminations of thereflecting surfaces due to debris from the radiation source or toreactions with gas remaining in the vacuum chamber during operation.Radiation sources for EUV lithography today are gas discharge plasmas orlaser plasmas. The substances used for plasma generation, however, canmove from the radiation source to the optical components and condense onthe optical surfaces, thereby reducing their reflectivity. The materialreleased from the radiation source and moving in the direction of theoptical components is called debris. Other contaminations of the opticalcomponents can result from the fabrication process, transport ormounting of the optical components. Furthermore, the reflectivity of thereflecting surfaces can be reduced by an increased surface roughness, areduced density or a reduced thickness of the reflecting layer orlayers, due to the operation of the radiation source.

WO 2004/092693 A2 discloses a method and apparatus for debris removalfrom a reflecting surface of an EUV collector in an EUV lamp. In thismethod, a controlled sputtering ion source is created which comprises agas with the atoms of the sputtering ion material and a stimulatingmechanism causing the atoms of the sputtering ion material to exit in anionized state. With this sputtering ion source the debris materialdeposited on the reflecting surfaces of the EUV collector is removed bysputtering. In order to avoid a removal of the top layer of thereflecting surface, the ionized state of the sputtering ion material isselected to have a distribution around a selected energy peak that has ahigh probability of sputtering the debris material and a very lowprobability of sputtering the material of the top layer of thereflecting surface.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of in situtreating an optical component reflecting EUV and/or soft X-ray radiationin an optical device as well as a corresponding optical device, whichallow an enhancement of a reduced reflectivity of the optical componentand at the same time enhance the lifetime of the optical component.

The object is achieved with the method and the optical device accordingto claims 1 and 10. Advantageous embodiments of the method and theoptical device are subject matter of the dependent claims or describedin the subsequent part of the description and examples.

In the proposed method of in situ treating an optical component in anoptical device, said optical component being arranged in a vacuumchamber of said optical device and comprising one or several reflectingsurfaces having a top layer of one or several surface materials, asource of said one or several surface materials is provided in saidvacuum chamber of said optical device and surface material from saidsource is deposited on said one or several reflecting surfaces duringoperation and/or during operation pauses of said optical device in orderto cover or substitute deposited contaminant material and/or tocompensate for ablated surface material.

In the proposed method, therefore, the one or several materials of thetop layer of the one or several reflecting surfaces are deposited onthese surfaces in situ, i.e. without disassembling the optical device.Due to this deposition of the surface materials, contaminations on thereflecting surface are covered by the surface materials, resulting in animproved reflectivity of this surface. Furthermore, since surfacematerial of reflecting surfaces in EUV lamps may be ablated duringoperation, this ablated material is also compensated by the materialdeposited with the proposed method. This means that the reflectinglayers of the optical devices will not lose their reflectivity due toerosion of these layers and therefore the lifetime of these opticalcomponents will be higher than the lifetime of optical components whichare not treated with the proposed method.

The deposition of the material(s) can be achieved by various knownmethods, for example by evaporating the material from the materialsource or by (e.g. metal-organic) chemical vapor deposition of thematerial (CVD/MOCVD) or by a sputtering technique, in which a sputtertarget including the surface material is provided in the vacuum chamber.Using a sputtering technique it is also possible to generate ions of thesurface material(s) with a high kinetic energy, so that some of theseions then replace the atoms or molecules of the contaminant material onthe reflecting surface.

The present method can be applied continuously during operation of theoptical device, after mounting and before the first use of the opticalcomponents of this optical device, repeatedly during operation or inoperation pauses of the device as well as dependent on the reduction ofreflectivity during operation. In the last case, the reflectivity of atleast one of said one or several reflecting surfaces preferably ismeasured continuously or repeatedly. The surface material then is onlydeposited when said reflectivity has decreased below a threshold value,which can be set by the operator. When measuring the reflectivity, it ispossible to measure the reflectivity in the EUV or soft X-ray spectralrange. It is also possible to measure the reflectivity in otherwavelength ranges, provided that the reflectivity in these wavelengthranges is also an indication of the reflectivity in the EUV and/or softX-ray radiation range. Nevertheless, also other types of measurementscan be performed in order to derive a reduction in reflectivity. Suchmeasurements can include the measurement of the gas composition in theoptical device, for example the proportion of mirror material in thegas, the use of a crystal balance, the use of a diffractometer, the useof energy dispersive X-ray analysis or the use of spectroscopicellipsometry.

In a preferred embodiment, the source of one or several surfacematerials is provided as a sputter target in the vacuum chamber of theoptical device. The deposition of said surface materials is thenperformed by sputter deposition using an appropriate sputtering gas, inparticular an inert gas like argon (Ar), which normally is used as abuffer gas during operation of the optical device. This gas can beionized by known means, for example by light (e.g., UV, VUV or EUV), bythe generation of microwaves around the optical components, by applyingan RF field between the target and the optical components or between theoptical components and a further electrode arranged in the vacuumchamber. Furthermore, also an ion gun can be used to provide thenecessary sputtering ions. The sputtering can be performed continuouslyor pulsed, with or without a magnetron unit or an additional reactivegas. Moreover, the application of an rf substrate bias to control ionbombardment and substrate cooling or heating to affect surface mobilityand diffusion are feasible.

The sputter target can be provided as a separate component arranged inthe optical device. In this case, the separate component preferably ismovable between a position close to the reflecting surface for sputterdeposition and away from this surface for normal operation of theoptical device. In a preferred embodiment, at least one of severalsputter targets is provided as a layer of the substrate material onsurfaces of optical components of the optical device, which surfaces arenot used for reflecting the EUV and/or soft X-ray radiation.

The proposed optical device has at least one optical component in avacuum chamber, which optical component has one or several reflectingsurfaces with a top layer of surface materials, e.g. Ru or Mo/Simultilayers. The optical device, which is preferably an EUV lamp, forexample for EUV lithography, comprises at least one source of said oneor several surface materials, said source being usable or operable todeposit surface material on said one or several reflecting surfacesduring operation and/or during operation pauses of said optical devicein order to cover or substitute deposited contaminant material and/or tocompensate for ablated surface material.

The optical device preferably also comprises the means for depositingsurface material from said source on the one or several reflectingsurfaces. These means preferably are electrical means for evaporatingsaid material(s) from said source or for ionizing a sputtering gas andsputtering said surface material(s) from a sputter target. To this end,the optical component or part of this optical component is connected tothe electrical means in order to apply an RF voltage between thereflective surface and the sputter target. Furthermore, also a DCvoltage can be applied to generate a DC bias at the reflecting surface.

Preferably, said source of surface materials is arranged to be movablebetween at least two positions inside said vacuum chamber of saidoptical device. One position is a position close to the reflectingsurface in order to achieve an optimized material deposition on thissurface. The other position is a position away from this surface inorder to achieve an operation of the optical device without disturbanceby the source.

The optical device can include a control unit controlling the depositionof said surface material(s) on the reflecting surfaces. Preferably,means for measuring the reflectivity of at least one of the reflectingsurfaces are provided in said optical device, wherein said control unitcontrols the means for depositing surface material in such a manner thatsaid means deposit said surface material(s) only when said reflectivityhas decreased below a preset threshold value. The control unit can alsocontrol the movement of the source of material(s) between the twopositions.

In a preferred embodiment, said optical device contains an EUV collectorhaving several shells for collecting EUV and/or soft X-ray radiationfrom a corresponding radiation source. In this embodiment, the frontsides of the collector shells represent the reflecting surfaces, whilethe back sides of these shells do not contribute to the reflection. Theback sides are covered with a thick layer of the surface material andserve as a sputter target for the sputter deposition. An additionaldummy shell is placed close to the reflecting surface of the inner layerof the collector and is also covered with the surface material on itsback side. The one or several surface materials from the back sides arethen sputtered according to the proposed method and deposit on the frontsides, i.e. on the reflecting surfaces, of the corresponding opposingcollector shells.

In the present description and claims, the word “comprising” does notexclude other elements or steps, and the indefinite article “a” or “an”does not exclude a plurality. Also, any reference signs in the claimsshall not be construed as limiting the scope of these claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The following embodiments show examples of the present method and of acorresponding optical device with reference to the accompanyingdrawings, without limiting the scope of the invention. The drawingsshow:

FIG. 1 a schematic view of the principle of the present method;

FIG. 2 a schematic view of the principle of sputter deposition as anexample of the present method;

FIG. 3 an example of the proposed optical device in a partial view; and

FIG. 4 a schematical configuration of an EUV irradiation unit.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present method and the corresponding device are explainedhereinbelow by means of an example of an EUV lamp which is used for EUVlithography. In such a lamp, hot plasma is generated to emit the desiredEUV radiation, which is focused by a collector and may be deflected byone or more further optical components. The collector in this examplecomprises several shells of grazing incidence mirrors having reflectingsurfaces made of a metallic ruthenium layer. The further opticalcomponents have a multilayer reflecting surface which is covered by aprotecting top layer of ruthenium. Although the proposed method isexplained by means of the example of ruthenium as a surface material, itis clear that the method is also applicable to other surface materialsused as reflecting or protecting layers in the EUV or soft X-rayspectral range.

During operation of such an EUV lamp, debris from the plasma source, forexample tin, escapes towards the optical components and may deposit onthe reflecting surfaces. The reflectivity of the collector and otheroptical components is reduced by this deposition of contaminantmaterial. After this reduction in reflectivity has reached a presetthreshold value, the reflective surfaces are in situ treated using theproposed method.

To this end, a ruthenium source 1 is arranged in the optical device awayfrom the optical path of the EUV lamp. Ruthenium source 1 is then movedclose to the reflecting surface 3 of the optical component 2 to betreated, as schematically indicated in FIG. 1. Ruthenium source 1 canbe, for example, a sputter target or an evaporator. The ruthenium isthen sputtered or evaporated from the source 1 and deposits on thereflecting surface 3. Contaminant material on this surface, for exampletin, is covered with the deposited ruthenium. In the same way, alsoablation of ruthenium material from the reflecting surface by erosionduring operation of the EUV lamp is compensated for by depositing theadditional ruthenium on this reflecting surface.

FIG. 2 shows an example in which the ruthenium source is a sputtertarget formed of a carrier substrate 4 covered by a thick target layer 5of ruthenium. The carrier substrate 4 and the optical component 2 areconnected to a power supply for generating DC and AC (RF) voltages.Carrier substrate 4 serves as the cathode and optical component 2 as theanode, as is generally known from the field of physical vapordeposition. Between anode and cathode a working gas, for example argon,is present. The argon atoms are ionized by the applied RF voltage andthe argon ions are accelerated, for example at 300 eV, towards thetarget layer 5 to discharge free ruthenium atoms at a few eV. In thepresent example, optical component 2 is shown with a plane reflectingsurface 3. For optical components with such an easily accessiblegeometry, as is often the case for example with multilayer mirrors, thesputter target can have the same design and can be moved in front of andoriented parallel to the reflecting surface 3 to be treated, as shown inFIG. 2.

If the applied RF and/or DC voltage is selected such that ruthenium ionsare generated, sputtering of the reflecting surface can be achieved, asa result of which the contaminant materials are removed from thissurface and substituted by the ruthenium ions. The sputter efficiency ofruthenium is approximately 1 at ion energies of about 1 keV. This meansthat for each ruthenium ion one atom of the contaminant material or oneruthenium atom of the top layer of the reflecting surface is removed.Consequently, after an appropriate treating time, a relatively pureruthenium layer is achieved.

At least one of several target layers of the ruthenium source can alsobe provided on non-reflecting surfaces of the optical components, i.e.surfaces which are not used during operation of the EUV lamp. FIG. 3shows such an example of an optical device, in which back sides of acollector 6 are used as the sputter target. EUV radiation emitted fromradiation source 11, a plasma source, is focused by the collector 6.Collector 6 comprises four collector shells 7. The front side 8 of eachshell 7 has a reflecting surface; the back side 9 is not used forreflecting radiation. The back sides 9 of the shells 7 of the collector6 are covered with a thick target layer of ruthenium, and the reflectingsurfaces of the front sides are also made of a ruthenium layer. Thetarget layers can be applied galvanically. In addition to the fourcollector shells 7, a dummy shell 10 is provided in front of the innershell. The dummy shell 10 has the same target layer on the back side asthe other shells; the front side of this dummy layer, however, does nothave any function.

Shells 7 of collector 6 are electrically isolated and connected to apower supply 12. This power supply 12 generates a direct current (bias)as well as an RF alternating current for generating plasma between theshells. A working gas, for example argon, is present between the shells.This working gas is used as a buffer gas during operation of the EUVlamp. The DC potentials U1, U2, U3 and U4 of the different shells areselected such that the front side 8 of each shell 7 is chargedpositively against the back side of the opposing shell. This means thatU0>U1>U2>U3>U4. During operation of the power supply 12, the argon gasis ionized between the shells, thereby sputtering the ruthenium atomsfrom the back sides, which ruthenium atoms then deposit on the frontsides of the shells, i.e. on the reflecting surfaces.

This procedure can be performed during normal operation of the EUV lamp.It is however preferred to perform this sputter deposition only duringoperation pauses of the EUV lamp. Furthermore, it is not in any casenecessary to generate the sputtering plasma between each of the shellsat the same time. It is also possible to treat the single shells oneafter another. In this case, the DC and RF voltages are applied at thesame time only between one pair of opposing shells and are subsequentlyswitched to another pair of opposing shells and so on.

FIG. 4 schematically shows a typical layout of an EUV lithography systemwith a corresponding EUV lamp. The EUV lamp basically consists of theradiation source 11, the collector 6 and multilayer mirrors 13 in avacuum vessel 14. The radiation emitted from the radiation source 11 iscollected by the reflective collector 6 and focused on an intermediatefocus 15. At the position of this intermediate focus 15 an apertureconnects the first volume 16 with a second volume 17 of the EUV lamp. Inthis second volume 17 the multilayer mirrors 13 are arranged to guidethe radiation from the intermediate focus 15 to the lithography mask(not shown) and the wafer substrate 18. In most EUV lithography systems,means 19 for debris mitigation are arranged between the radiation source11 and the collector 6. In such an EUV lamp, the collector 6 can bedesigned as described in connection with FIG. 3 in order to continuouslyor repeatedly enhance the reduced reflectivity of the reflectivesurfaces.

LIST OF REFERENCE SIGNS

-   1 ruthenium source-   2 optical component-   3 reflecting surface-   4 carrier substrate-   5 target layer-   6 collector-   7 shell-   8 front side-   9 back side-   10 dummy shell-   11 radiation source-   12 power supply-   13 multilayer mirrors-   14 vacuum vessel-   15 intermediate focus-   16 first volume-   17 second volume-   18 substrate-   19 means for debris mitigation

1. Optical device for EUV and/or soft X-ray radiation, in particular anEUV and/or soft X-ray lamp, comprising at least one optical component ina vacuum chamber, which optical component has one or several reflectingsurfaces with a top layer of one or several surface materials, whereinthe optical device comprises at least one source of said one or severalsurface materials, said source being usable to deposit surface materialsin-situ on said one or several reflecting surfaces during operationand/or during operation pauses of said optical device in order to coveror substitute deposited contaminant material and/or to compensate forablated surface materials.
 2. Optical device according to claim 1,wherein said optical device comprises means for depositing surfacematerials from said source on said one or several reflecting surfacesduring operation and/or during operation pauses of said optical device.3. Optical device according to claim 1, wherein said source is arrangedto be movable close to said one or several reflecting surfaces inoperation pauses of said optical device.
 4. Optical device according toclaim 2, wherein said means for depositing surface materials comprise achemical vapor deposition system.
 5. Optical device according to claim1, wherein said source of surface materials is a sputter target and saidoptical device comprises means for depositing surface material from saidsource on said one or several reflecting surfaces by sputter deposition.6. Optical device according to claim 5, wherein said means fordepositing surface materials include a DC and RF power supply connectedto said source of surface materials and to said optical component. 7.Optical device according to claim 5, wherein said sputter targetcomprises a target layer (5) on one or several non-reflecting surfacesof said optical component (2, 6, 13) or of other optical components (6,13) of said optical device.
 8. Optical device according to claim 7,wherein said optical component is a collector having several collectorshells and said target layer is provided on back sides of said collectorshells.
 9. Optical device according to claim 1, wherein said opticalcomponent is a collector formed of one or several multilayer mirrors.10. Optical device according to claim 2, wherein said optical devicecomprises means for measuring a reflectivity of at least one of said oneor several reflecting surfaces continuously or repeatedly and a controlunit for controlling said means for depositing surface material in sucha manner that said means deposit said surface material only when saidreflectivity has decreased below a threshold value.
 11. Optical deviceaccording to claim 1, wherein said source of surface materials is asputter target and said optical device comprises means for depositingsurface material from said source on said one or several reflectingsurfaces by sputter deposition.
 12. Optical device according to claim11, wherein said sputter target comprises a target layer on one orseveral non-reflecting surfaces of said optical component or of otheroptical components of said optical device.
 13. Optical device accordingto claim 1, wherein said optical device comprises means for depositingsurface materials from said source on said at least one reflectingsurfaces during operation and/or during operation pauses of said opticaldevice.